In typical commercial electrographic printing or reproduction apparatus (electrographic copier/duplicators, printers, or the like), a latent image charge pattern is formed on a uniformly charged charge-retentive or photoconductive member having dielectric characteristics (hereinafter referred to as the dielectric support member). Pigmented marking particles (for example, toner) are manipulated into close proximity with the latent image charge pattern by a one or more development stations, allowing the pigmented marking particles to be attracted to the latent image charge pattern in order to develop such image on the dielectric support member. A receiver member, such as a sheet of paper, transparency or other medium, is then brought directly or indirectly via an intermediate transfer member, into contact with the dielectric support member, and an electric field is applied to transfer the marking particle developed image to the receiver member from the dielectric support member. After transfer, the receiver member bearing the transferred image is transported away from the dielectric support member, and the image is fixed (fused) to the receiver member by heat and/or pressure to form a permanent image thereon.
The development system of an electrophotographic printing or reproduction apparatus is ideally designed to provide a uniform toner concentration to the passing dielectric support member so that a uniformly charged latent image on the dielectric support member will be developed with a proportionally uniform density of toner. Unfortunately, many existing electrophotographic development stations produce “banding”, which is a noticeable and an undesirable variation in image density that manifests as alternating or varying density bands in areas which otherwise are supposed to have a uniform density.
Previous attempts at preventing or reducing banding have sometimes increased the overall developed image density (for example by increasing toner concentration) to a point where any variation in developed density is unnoticeable in dark image regions since the image appears uniformly saturated to a viewer, thereby hiding banding effects in those dark regions. Unfortunately, such techniques can lead to dusting and/or unwanted background toner in light or white areas.
Other attempts to reduce banding have focused on trying to manipulate or control one or more development station electrical biases to compensate for banding. For example, U.S. Pat. No. 6,101,357 discloses a method for reducing power-supply-induced banding by modulating an AC oscillation voltage in a way which minimizes electrical energy in a frequency spectrum that was found to contribute to certain kinds of banding. Similarly, U.S. Pat. No. 7,280,779 discloses a method of measuring an electrical potential on the surface of a developer roll and adjusting a time varying component of a voltage applied to the developer roller using the measured potential in order to reduce variation in the electrical potential and thereby reduce banding.
Further attempts at reducing banding have focused on trying to minimize vibrations or movement of the development station relative to the dielectric support member since variations in development station spacing can cause banding. For example, U.S. Pat. No. 6,236,820 discloses an imaging system which physically links key image subsystems to the one or more development stations in a way which minimizes their movements relative to each other, thereby reducing banding.
Still further attempts at reducing banding have shied away from identifying and addressing a root cause for the banding and instead have taken measures to introduce system noise in attempt to mask banding effects. For example, U.S. Pat. No. 6,567,110 discloses a method for coupling a noise generator to a component in a laser imaging assembly in order to create noise in the pre-developed latent image. Unfortunately, while the noise in the latent image may help obfuscate development banding effects, it is merely masking a problem and necessarily adds system cost through the need of yet more subsystem components.
Furthermore, while previous attempts to prevent development station banding may have focused on modifying the electrical development bias, the persistence of banding under a variety of bias conditions indicates that development station banding still remains an incompletely understood problem which requires further study.
Therefore, it would be beneficial if there were an inexpensive, yet reliable, method and system for reducing development station banding that could easily be implemented.